Adaptive body bias may be used after fabrication to improve a bin split in microprocessors and to reduce a variation in frequency and leakage caused by process variations. In performing adaptive body bias, a unique body bias voltage may be set to maximize the frequency of the processor subject to leakage and total power constraints and the type of transistor technology in use. Body bias voltages may be applied to processors and other circuits that use PMOS transistors, NMOS transistors, or both.
Two types of body bias voltages may be used to control the frequency of a microprocessor, namely forward body bias (FBB) voltage and reverse body bias (RBB) voltage. If forward body bias (FBB) is used, the frequency of the processor may increase along with leakage. If reverse body bias (RBB) is applied, the frequency and leakage of the processor may decrease. In some circuits, both forward and reverse body bias voltages are applied in order to compensate for process variations within the die. Parts of the circuit that are too slow may receive forward body bias to increase their speed, while other parts that are faster than necessary may receive reverse body bias to reduce their leakage power. Because the effectiveness of RBB is diminishing with process scaling and because leaky dies may be recovered better by lowering the Vcc, an adaptive body bias technique that uses FBB may be attractive.
The circuitry for applying adaptive body bias may include two blocks, namely a central bias generator (CBG) and a local bias generator (LBG). The central bias generator may generate a reference voltage that is process, voltage and temperature independent. This voltage may represent the desired body bias to apply to transistors in the microprocessor core or other locations. If both PMOS and NMOS transistors are to be biased, then two central bias generators may be used each generating a different reference voltage for each transistor type. Alternatively, a single central bias generator may be used that is capable of generating the reference voltages for both transistor types.
On the other hand, many local bias generators may be distributed throughout a processor die. The local bias generators may translate the reference voltage from the CBG into local block supply voltages and then drive these voltages to the transistors or other devices in each respective block. The translation may ensure that if a local block supply voltage changes, the body bias will change at substantially a same time so that a constant bias is maintained. For example, for NMOS body bias, the body voltage may track any variation in the local block ground (Vss).